The formation of extremely small solder bumps (balls) is a very important process in the production of many types of electronic devices, and is particularly important for flip-chip mounting of semiconductor chips on highly integrated cards and boards, chip carriers, etc. Therefore, various means for achieving such formation have been devised and utilized in the industry. Exemplary methods include a solder plating process and a solder powder bonding method. However, these methods have not come into widespread use, because the methods require large scale apparatus and suffer from limitations on the bump density and type of solders that can be used.
One method that imposes a less strict limitation on the solder type and ensures high density bump formation is a solder injection method as defined in U.S. Pat. No. 5,238,176 and TOKKOHEI (publication of the Japanese Patent Application) No. 6-66314 (1994)). In accordance with this method, a metal mask is prepared which has a hole conformable to a pad formed on a target substrate, and a chamber is formed with metal masks at its bottom. See FIGS. 14(a) to 14(d) of the instant drawings, which illustrate this approach. Solder melt is retained on the metal mask in the chamber. At this time, the solder melt does not flow down through the hole between the metal masks due to its surface tension. After the metal mask is positioned in registration with the pad on the substrate, a gas is fed into the chamber to pressurize the solder melt, whereby a portion of the solder melt is extruded through the hole. Thus, the solder melt portion can be placed on the pad of the substrate in an amount determined by the physical dimension of the metal mask. In turn, the pressure of the gas is reduced, so that the solder melt portion in the hole between the metal masks is separated from the solder melt in the upper chamber by the surface tension of the solder melt and an attraction force between the solder melt portion and the pad. Thus, uniform bumps are formed on the pads. However, this method of forming solder bumps requires a metal mask which is formed with minute holes in conformity with each terminal pattern of a chip, resulting in higher production costs. Further, the metal mask formed with the minute holes must be precisely positioned with respect to the chip's terminal pattern. Therefore, the time required for an adjustment operation for the positioning is prolonged, thereby reducing productivity.
Methods of physically applying an appropriate amount of a solder onto target pads are known, though these methods are not intended for solder bump formation. With reference to FIGS. 15(a) to 15(d) of the instant drawings, one example of a physical application method is to allow a drop of a solder melt to flow over the pads. In accordance with this application method, portions of the solder melt adhering onto the pads are mostly forced off and absorbed in the drop because the drop of the solder melt has a greater surface tension. As a result, the solder melt portions remain as thin films on the pads. Therefore, this method cannot be applied to solder bump formation. This phenomenon is also observed in a solder wave operation and solder dip processes, which are also not readily adaptable to solder bump formation.